



mrmr'inifiA 


Df;PARTMENT OF THE INTERIOR 

UNITED STATES GEOLOGICAL SURVET 

GEORGE OTIS SMITH, Director 


Bulletin 580—B 


NOTES ON THE 


IINAWEEP COPPER DISTRICT, C0I/)RA1)0 



BY 


B. S. BTTTT.ER 




CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1913, PART I—B 




m 







WASHINGTON 

GOVERNMENT PRINTING OFFICE 

. 1911 


Monograph 























































DEPARTMENT OF THE INTERIOR 

UNITED STATES GEOLOGICAL SURVEY 

GEORGE OTIS SMITH, Director 


BULiLETIN 580—B 


NOTES ON THE 

UNAWEEP COPPER DISTRICT, COLORADO 


B> S. BUTLER 


CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1913, PART I—B 




WASHINGTON 

GOVERNMENT PRINTING OFFICE 

1914 











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•f. S' 





d; of 0, 

APR 23 i914 


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J^OTES ON THE UNAWEEP COPPER DISTRICT 

COLORADO. 


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By B. S. ButIuER. 


INTRODUCTION. 


The following notes are the result of a visit of two days to the 
Una weep district, in September, 1913. The time spent was alto¬ 
gether too short for a detailed study of the geology, and owing to 
the meager development and the fact that most of the properties 
were idle opportunity for a very satisfactory study of the ore de¬ 
posits was lacking. The geology, however, is relatively simple, and 
it is believed that the more important features bearing on the ore 
deposits are recorded in the following paragraphs. The writer 
wishes to acknowledge assistance and courtesies from Mr. J. S. Shaw, 
of Grand Junction, and Mr. F. B. Grant. 

The Unaweep district is located in Mesa County, Colo., 12 to 15 
miles west of AYliitewater, a station on the Montrose branch of the 
Denver & Rio Grande Railroad. The wagon road to the district has 
rather heavy grades for the first 8 to 9 miles, beyond which it fol¬ 
lows Unaweep Canyon with moderate grade. The heavy grades are 
due to the fact that the road passes over a spur of the mesa instead 
of following the Unaweep Canyon, but a road following the grade 
of the canyon could be constructed. The bottom of Unaweep Can¬ 
yon in the central portion of the district has an elevation of about 
6,250 feet. Most of the prospects are several hundred feet above 
the bottom of the canyon. A small stream in the canyon furnishes 
water for irrigating a few ranches devoted mainly to the raising of 
alfalfa. Several of the prospects have developed sufficient water 
for uses other than domestic, and the side canyons have small springs 
that furnish w^ater for a part of the year. 


TOPOGRAPHY AND PHYSIOGRAPHY 


Topographically and physiographically the district is typical of 
the plateau country. Unaweep Canyon is a great cut through the 
plateau from the canyon of Gunnison River on the east to that of 
Dolores River on the west. The divide between the two drainage 


29262°—14 


19 




20 CONTRIBUTIONS TO ECONOMIC GEOLOGY^ 1913, PART I. 

systems in this canyon is exceptionally low. In the vicinity of the 
mines the lower 300 feet of the canyon is in granite, the walls rising 
steeply from the flat bottom. Extending back from the upper edge 
of the granite is a bench varying from less than 100 to 200 or 300 
yards in width. The floor of this bench, is the surface of the granite 
from which the overlying sediments have been removed. Immedi¬ 
ately back of the granite bench is a short talus slope formed from a 
soft shale overlying the granite, and above this rises a nearly per¬ 
pendicular wall of red sandstone. In the vicinity of the mines this 
sandstone is about 200 feet thick, but is said to attain a thickness of 
400 feet at some localities. On the top of the red sandstone is a 
second bench bounded by cliffs of gray sandstone, which was rnot 
examined, but which has a thickness of several hundred feet. 

The granite bench renders building of roads to the prospects 
relatively easy, as most of the prospects have been developed on this 
bench or but slightly above it, and few of the side canyons have been 
cut back across the bench to the base of the red sandstone cliff. 

GEOLOGY. 

IGNEOUS ROCKS. 

The district contains both igneous and sedimentary rocks. The 
basal formation is a granite that includes irregular blocks of mica 
and hornblende schist. The granite varies considerably in physical 
and chemical properties. Much of it is rather uniformly medium 
grained and is composed of feldspar, quartz, muscovite, and biotite, 
recognizable in the hand specimen. Under the microscope apatite, 
rutile, zircon, and magnetite are recognized as accessory minerals. 
The feldspar is mainly microcline but includes a little plagioclase 
near albite in composition. The muscovite is as a rule considerably 
more aboundant than the biotite, though there is considerable varia¬ 
tion in the mica content and in the relative proportion of the two 
varieties. 

At some places the granite is very much coarser, containing feld¬ 
spar crystals as much as an inch in greatest dimension. The coarser 
type appears to be more siliceous, quartz and pink feldspar being 
more abundant and the amount of mica being correspondingly de¬ 
creased. In one specimen examined microscopically titanite is rather 
abundant. Between the finer and coarser type there are apparently 
all gradations. 

The irregular bodies of schist inclosed in the granite are for the 
most part mica and hornblende schists, but some of them are very 
siliceous. 

The granite and schist are cut by a great number of pegmatite 
dikes, which range in size from a few inches to several feet. The 
dike rocks are usually very coarsely crystalline and are composed 


UNAWEEP COPPEE DISTEICT, COLO. 


21 


essentially of quartz, alkali feldspar, and muscovite. In the larger 
dikes the individual crystals of the minerals may reach several inches 
in greatest dimension, and lenses of quartz that extend a few feet 
along the strike and are 6 inches or more in thickness are not uncom¬ 
mon. The granite is also cut by dikes of diabasic rock. The 
largest of these dikes observed, in Taylor Gulch, is said to have 
an average thickness of 200 feet or more. The writer did not con¬ 
firm this measurement, but the dike seems to have fully this thickness 
at some points. The dike on the Chance and Bell claims is 12 to 14 
feet thicky and the one on the McKinley claim is reported to be of 
the same thickness, and is generally supposed to be the same dike. 
There are numerous other dikes of various sizes in the district. 

The composition of the dikes varies somewhat, but they are all dia¬ 
base composed essentially of plagioclase, augite, and magnetite. The 
rocks have been considerably altered, and an accurate determination 
of the composition of the feldspar is not possible, but it is probably 
close to andesine. In most of the dikes the augite has been largely 
serpentinized. 

SEDIMENTARY ROCKS. 

The sedimentary rocks of the area consist of shales and sandstones. 
Lying immediately on the granite is 50 to 70 feet of red ferruginous 
shale which breaks down readily, forming a talus that obscures the 
immediate contact. In some places there seemed to be a few inches 
of a rather fine conglomerate immediately above the granite, but at 
no point observed was the actual contact of granite and sediments 
well exposed. Overlying the red shale is 200 feet of*red sandstone 
with beds of fine conglomerate. Above the red sandstone are beds 
of white to gray sandstone, the thickness of which was not estimated. 

No fossils were collected in the district, and the formations were 
not traced into areas where the age of the sediments has been deter¬ 
mined. The beds above the pre-Cambrian farther west in Unaweep 
Canyon are considered by Cross ^ to be equivalent to the Dolores and 
La Plata formations of the San Juan region, and the rocks in the 
Unaweep district are probably to be correlated with these formations. 

RELATION OF SEDIMENTARY AND IGNEOUS ROCKS. 

The sedimentary rocks at all points where they were observed 
appear to have been deposited on the granite, and as no dikes were 
found in the sediments all the igneous rocks appear to be older than 
the sedimentary rocks. The exact age of the crystalline rocks can 
not be stated, but there is little doubt that they are of pre-Cambrian 
age and that they had been deeply eroded before they were covered 
by the sediments.^ 

1 Cross, V liitman, Jour. Geology, vol. 15, No. 7, p. 648, 1907 

2 Idem, p. 676. 





22 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1913, PART I. 

STRUCTURE. 

The structure of the district is relatively simple. When observed 
locally the sedimentary beds appear essentially horizontal. As one 
goes up the valley, however, the surface of the granite is seen to rise 
from the valley bottom to fully 350 feet above the bottom in a dis¬ 
tance of about 3 miles. The dip of the strata is therefore consid¬ 
erably greater than the grade of the stream. The granite surface 
apparently has a dip of 4°-5° NE. 

Some Assuring and faulting have occurred in the district. Many 
of the dikes, both the pegmatites and the basic dikes, strike about 
N. 70°-80° W. (magnetic) and dip very steeply. These dikes have 
evidently occupied fissures. There is also a prominent jointing in the 
granite having the same general direction. 

The fissures that have been occupied by the dikes were apparently 
formed prior to the deposition of the sedimentary rocks, soon after 
the granite was solidified, but along some of the prominent fissures 
there has been movement that has faulted the sedimentary rocks. 
Such faulting was noted near the McKinley mine, and a fault with 
GO to 70 feet throw crosses the canvon 1 to 14 miles east of Grant’s 
ranch. There has been considerable prospecting along this fault. 
It strikes in the same general direction as the others and is probably 
an old fissure which has been reopened. 

ORE DEPOSITS. 

Copper is the main valuable metal in the district, though the 
ores contain some gold and silver. It is reported that several car¬ 
loads of copper ore were shipped about 10 years ago. A matte 
smelter was erected in the district some years ago, and a few tons of 
ore treated. 

The ore deposits occur in fissures that cut both the igneous and 
sedimentary rocks. Several of the ore fissures are apparently due 
to later movements along the fissures in which the basic dikes were 
intruded,'and thus there is apparently a close relation between these 
dikes and the ore deposits, though basic dikes are not associated with 
all the deposits. 

In the ore deposits associated with basic dikes the vein usually 
lies along one wall of the dike, between the dike and the granite. 
The other deposits are distinct fissure veins with granite or shale 
walls, or where faulting has occurred granite may form one wall and 
shale the other for short distances. Most of the prospecting has been 
done in the igneous rocks, though on the Nancy claim ore occurs in 
the shale a short distance above the granite. Alteration of the mas¬ 
sive igneous rocks along fissures has usually been slight, but where 
the fault fissures have cut the sandstones there has been a marked 


UNAWEEP COPPER DISTRICT^ COLO. 


23 


addition of silica, cementing the sandstones into a resistant quartzite 
for several feet from the fissure, as is well illustrated 1 to IJ miles 
east of Grant’s ranch. 

The prevailing vein minerals are calcite, quartz, and a little 
fluorite, together with pyrite and chalcopyrite and in some deposits 
rather abundant hematite. 

Oxidation has not been very extensive, as most of the ore contains 
a rather large percentage of primary sulphide, even in the shallow 
workings. The oxidation has resulted principally in the formation 
of limonite and malachite. A little chalcocite was noted, but there 
has apparently been very little sulphide enrichment. The copper 
sulphate resulting from the oxidation of chalcopyrite has apparently 
reacted with the calcite of the gangue to form the relatively stable 
copper carbonate. There seems no reason for believing that there 
has been any considerable migration of the copper or that in the 
district in general richer or larger ore bodies than those near the 
surface are to be expected at greater depth. It is of course possible 
that in an individual deposit the outcrop may chance to be a narrow 
or lean portion of the vein and that it will be found to be larger and 
richer at greater depth. It should be borne in mind, however, that^ 
the reverse is quite as likely to be true. 

There has been considerable prospecting in the district, but most 
of the properties were idle at the time of visit, and it is therefore 
impossible to give an accurate description of the different prospects. 
The deepest mine is the McKinley, which has been developed by 
a shaft said to be about 600 feet deep. It is stated that at a depth 
of about 150 feet a small body of ore was encountered. The Kancy 
is developed by two shafts, each reported to be more than 100 feet 
deep, and by a tunnel about 300 feet long. A shoot of ore was found 
near the surface and was followed to a depth of more than 50 feet, 
and the writer was informed by Mr. J. S. Shaw that 21 cars of ore 
averaging 16 per cent of copper were shipped from this deposit. 
The ore on the dump contains pyrite and chalcopyrite with abundant 
hematite. 

The Chance claim is developed by a tunnel 700 feet long connect¬ 
ing with a shaft more than 300 feet in depth. The tunnel follows a 
basic dike and disclosed small bodies of vein minerals between the 
dike and the granite walls. 

The Bell claim is believed to be on the same vein as the Chance 
and has been developed by a shaft, said to be about 120 feet in depth, 
which encountered some ore. There are numerous other claims in the 
district on which some development work has been done and in 
which some ore has been found. 


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